KR20190078259A - High strength cold rolled steel sheet having low mechanical properties deviation, good stretch flangeability and high recovery rate - Google Patents

Abstract

The present invention provides a high-strength cold-rolled steel sheet having low material property deviations and excellent stretch flangeability and recovery rates and a manufacturing method thereof. According to an embodiment of the present invention, the high-strength cold-rolled steel sheet comprises 0.030-0.080 wt% of C, 1.5-3.0 wt% of Mn, 0.03-0.50 wt% of Si, 0.001-0.045 wt% of P, 0.0005-0.0035 wt% of S, 0.5-2.5 wt% of Cr, 0.05 wt% or less of Al, 0.005-0.10 wt% of Ti, 0.005-0.10 wt% of Nb, 0.0005-0.004 wt% of B, 0.001-0.010 wt% of N, and the remainder consisting of Fe and inevitable impurities, and satisfies the following relations 1-4. The microstructure includes 30-70 area% of ferrite and 30-70 area% of the sum of martensite and autotempered martensite. The relation 1 is [C] <= [Cp], the relation 2 is 1.5 <= [C×Cp]×1000 <= 3.5, the relation 3 is 165 <= [Ceq]×1000 <= 235, and the relation 4 is 50 <= [Ceq]/([C×Cp]) <= 150, wherein [Cp] in the relations 1-4 is 0.09-0.038C-0.013Mn-0.028Si+0.00015Cr, [Ceq] is C+Si/30+Mn/20+2P+3S, and the content C, Mn, Si, Cr, P and S each represents wt%.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength cold rolled steel sheet having a low material deviation,

The present invention relates to a high-strength cold-rolled steel sheet having a small material variation, excellent stretch flangeability, and a high yield rate, and a method for producing the same.

In order to regulate CO ₂ emissions and improve fuel efficiency worldwide, automakers are constantly demanding a lightweight body. In order to reduce the weight of automotive steel sheet, the thickness of the steel sheet must be lowered. On the other hand, there is an inconsistent aspect that the thickness of the steel sheet must be increased to secure collision safety.

In order to solve these contradictory aspects, it is necessary to increase the formability while increasing the strength of the material. This is because the dual phase steel (DP steel) known as AHSS (Advanced High Strength Steel), Transformation Induced Plasticity Steel, TRIP Steel), and Complex Phase Steel (CP Strong).

Molding modes of automobile parts are divided into extrusion molding, stretch flange molding, drawing and bending. It is very important to secure proper moldability as well as strength in order to satisfy these requirements. In particular, among the various molding modes used in automotive steel sheets, the Hole Expanding Ratio is a mechanical characteristic as important as tensile strength and elongation. It is also known that stretch flangeability is more dependent on microstructure than mechanical properties. The stretch flangeability is defined as the ultimate strain that can withstand a large deformation exceeding the stretch.

In order to achieve excellent elongation flange performance in the 590 / 780MPa cold-rolled steel sheet, which is widely applied to automobile parts, the case of the martensite single phase is better than that of the two-phase structure steel in which martensite and ferrite are uniformly distributed It is known that, in the case of two-phase structure steels, the fraction of martensite increases, and when the difference in hardness between phases decreases, excellent stretch flangeability is exhibited. However, when the martensite fraction increases, there is a problem that the elongation rate is decreased and the other molding mode can be inferior.

In general, the AHSS of 780 MPa or more has a problem of increasing the rolling load during cold rolling since the alloy element and the added amount are increased to secure high strength and ductility / formability at the same time, so that the shape of the strip is limited, <1.0 mm). Particularly, in the batch process of existing hot-rolled mill, since the top portion is inserted in the rolling mill for each coil and the tail portion must exit from the rolling mill, this portion must be cut to open the thickness deviation and scale quality to the top and tail portions It is difficult to manufacture hot rolled steel sheets because of a very low yield and a high risk of plate fracture, so that there are many restrictions on the thickness of the final cold rolled steel sheet.

Patent Literatures 1 and 2 disclose technologies related to the manufacture of such AHSS steel, but all of these methods relate to a method of manufacturing in a conventional hot-melt mill, and it is difficult to avoid the problem that a material deviation in a production line is largely generated in a width and a length direction It is true. This is because, in the batch process of conventional hot-rolled mill, the top portion is drawn into the rolling mill for every coil, and the tail portion must exit the rolling mill. Therefore, this portion must be cut in order to open the thickness variation and scale quality to the top and tail portions Since the rate of realization is very low and the risk of sheet breakage is high, it is difficult to manufacture hot rolled steel sheets, and therefore, there are many restrictions (? 1.0 mm) in securing the thickness of the final cold rolled steel sheet.

Therefore, it is required to develop a new manufacturing process capable of overcoming the above-mentioned problems. In addition to this, it is urgently required to develop a manufacturing process capable of manufacturing a high strength cold rolled steel sheet having a small variation in material, excellent stretch flangeability and a high water-rejection ratio, in accordance with strong demands of the passenger collision stability and CO ₂ environment regulation worldwide.

U.S. Patent No. 4285741 U.S. Patent No. 4,255,751

One aspect of the present invention relates to a high strength cold rolled steel sheet having a small variation in material, excellent elongation flangeability, and a high yield rate by deriving optimal alloy components and process conditions as well as a continuous continuous rolling mode in a performance- .

An embodiment of the present invention is a steel sheet comprising, by weight%, 0.030 to 0.080% of C, 1.5 to 3.0% of Mn, 0.03 to 0.50% of Si, 0.001 to 0.045% of P, 0.0005 to 0.0035% of S, 0.001 to 0.04% N, 0.001 to 0.010% N, 0.001 to 0.010% of B, and the balance of Fe and other unavoidable impurities, 4, and the microstructure has an area fraction of 30 to 70% for ferrite and 30 to 70% for martensite and autotemplated martensite: 30 to 70%, and has excellent stretch flangeability and excellent water- .

[Relation 1] [C]? [Cp]

[Relation 2] 1.5? [C 占 Cp] 占 1000? 3.5

[Relation 3] 165? [Ceq] x 1000? 235

[Relation 4] 50? [Ceq] / ([CxCp])? 150

 (Cp) in the above relational expressions 1 to 4 is 0.09-0.038C-0.013Mn-0.028Si + 0.00015Cr, [Ceq] is C + Si / 30 + Mn / 20 + 2P + 3S, , And the contents of Cr, P, and S respectively represent% by weight.)

In another embodiment of the present invention, there is provided a steel sheet comprising: 0.030 to 0.080% of C, 1.5 to 3.0% of Mn, 0.03 to 0.50% of Si, 0.001 to 0.045% of P, 0.0005 to 0.0035% of S, 0.001 to 0.04% N, 0.001 to 0.010% N, 0.001 to 0.010% of B, and the balance of Fe and other unavoidable impurities, 4 is continuously cast to obtain a thin slab; Subjecting the thin slab to rough rolling to obtain a bar; Hot rolling the bar at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet; And winding the hot-rolled steel sheet at 400 to 650 ° C, wherein each of the steps is performed continuously, cold-rolling the rolled hot-rolled steel sheet to produce a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet in a temperature range of Ac3-50 DEG C to Ac3 + 30 DEG C; A primary cooling step of cooling the continuously annealed cold rolled steel sheet to a temperature range of Ar1 to Ar3 at an average cooling rate of 1 to 10 占 폚 / s; And a second cooling step of cooling the primary cold-rolled steel sheet to a temperature range of Mf-50 ° C to Ms + 50 ° C at an average cooling rate of 5 to 30 ° C / s, and before and after the hot- The overlap area ratio of the cooling water injected into the bar in the rolling scale breaker satisfies the following relational expression 5, the deviation in temperature during the hot rolling is 75 占 폚 or less, the deviation in rolling speed is 60mpm or less, A method of manufacturing a high strength cold rolled steel sheet excellent in the rate of water loss.

[Relation 1] [C]? [Cp]

[Relation 2] 1.5? [C 占 Cp] 占 1000? 3.5

[Relation 3] 165? [Ceq] x 1000? 235

[Relation 4] 50? [Ceq] / ([CxCp])? 150

[Relation 5] (X ³ -85X ² + 1800X) / 1000? 5

 (Cp) in the above relational expressions 1 to 4 is 0.09-0.038C-0.013Mn-0.028Si + 0.00015Cr, [Ceq] is C + Si / 30 + Mn / 20 + 2P + 3S, , The contents of Cr, P, and S respectively represent% by weight, and in the above-mentioned relational expression 5, X means the overlap area ratio (%) of the cooling water sprayed on the steel sheet from the finish rolling scale breaker.

According to an aspect of the present invention, there is provided a high-strength cold-rolled steel sheet having a small variation in material, excellent elongation flangeability, and a high yield rate by deriving optimum alloy components and process conditions while using continuous continuous rolling mode in a performance- Method can be provided.

In addition, since the steel sheet is manufactured in the continuous continuous rolling mode in the performance-rolling direct rolling process, there is an advantage in that the steel sheet is excellent in the thickness deviation and the surface quality without the top and tail portions, so that the material deviation is small and the slip rate is high.

In addition to this, it is possible to omit the reheating step in the conventional hot melt mill through the thin slab method, thereby saving energy and improving the productivity. In addition, it is possible to use the steel in which the scrap of scrap iron is dissolved in the electric furnace through the thin slab reclamation method, thereby enhancing the recyclability of resources.

1 is a schematic view of a facility for a performance-rolling direct process that can be applied to the present invention.
2 is another schematic diagram of a facility for a performance-rolling direct process that can be applied to the present invention.
3 is a schematic diagram showing one embodiment of a Finishing Mill Scale Breaker (FSB).
4 is an optical microscope tissue photograph of the inventive example 1 according to an embodiment of the present invention.
5 is a SEM micrograph of Inventive Example 1 according to an embodiment of the present invention.
FIG. 6 is a transmission electron microscope (TEM) micrograph of the inventive example 1 according to an embodiment of the present invention. FIG. 6 is a TEM micrograph of 50,000 times on the left side and 100,000 times magnification of the [X] and [Y] It is an organization photograph.
7 is a graph showing the ferrite (F) uniaxial crystal grain size distribution of Inventive Example 1 according to an embodiment of the present invention.
FIG. 8 is a graph showing a single-axis grain size distribution of martensite (M) + auto-temperate martensite (AM) of Inventive Example 1 according to an embodiment of the present invention.
9 is a photograph of a precipitate of Inventive Example 1 taken by a transmission electron microscope (TEM) according to an embodiment of the present invention.
10 is a graph showing a distribution of size of precipitates of Inventive Example 1 according to an embodiment of the present invention.
FIG. 11 is a graph showing the distribution of the distance between precipitates of Inventive Example 1 according to an embodiment of the present invention.

Hereinafter, the present invention will be described. First, the alloy composition of the present invention will be described.

C: 0.030 to 0.80 wt%

Carbon (C) is a very important element for increasing the strength of a steel sheet and securing a composite structure composed of ferrite and martensite. When the C content is less than 0.030%, it may be difficult to secure the desired strength in the present invention. On the other hand, when the C content is more than 0.080%, an apodization reaction (L + Delta Ferrite → Austentite) occurs at the time of solidification of molten steel, so that a solidified cell having a non-uniform thickness is formed and molten steel outflow may occur. Therefore, the C content is preferably 0.030 to 0.080%.

Mn: 1.5 to 3.0 wt%

Manganese (Mn) is an element that has a very strong effect of solid solution strengthening and at the same time promotes the formation of composite structure composed of ferrite and martensite. When the Mn content is less than 1.5%, it may be difficult to obtain the intended strength in the present invention. On the other hand, when the Mn content is more than 3.0%, it is difficult to obtain the desired elongation, and the weldability and hot rolling property may be deteriorated. In addition, if the Mn content is excessively added, since a delta-ferrite region is reduced at a temperature near the coagulation, an apodization reaction may occur even at a low C region, so that a coagulation cell having an uneven thickness is formed at high- Leakage of molten steel may lead to industrial accidents. Therefore, the Mn content is preferably 1.5 to 3.0%.

Si: 0.03 to 0.50 wt%

Silicon (Si) is a useful element that can secure strength without deteriorating the ductility of the steel sheet. It is also an element promoting the formation of martensite by promoting ferrite formation and promoting C concentration in untransformed austenite. When the Si content is less than 0.03%, it is difficult to sufficiently secure the above effect. On the other hand, when the Si content is 0.50% or more, the scale is generated on the surface of the steel sheet, traces remain on the surface of the steel sheet after pickling, and the surface quality may be deteriorated. Therefore, the Si content is preferably 0.03 to 0.50%.

P: 0.001 to 0.045 wt%

Phosphorus (P) is an element showing the effect of strengthening the steel sheet. When the P content is less than 0.001%, it is difficult to secure the effect. On the other hand, when the P content exceeds 0.045%, the grain boundary and / or the intergranular grain boundary may be segregated to cause brittleness, and the mechanical properties of the welded portion may be deteriorated. Therefore, the content of P is preferably 0.001 to 0.045%.

S: 0.0005 to 0.0035 wt%

Sulfur (S) is an impurity which segregates during MnS nonmetal inclusions and performance solidification in steel and can cause high-temperature cracks, and the mechanical properties of welds may deteriorate. Therefore, the content should be controlled as low as possible, and it is preferable to control the content to 0.035% or less. In order to suppress the increase in cost due to desulfurization, the lower limit is preferably 0.0005%.

Cr: 0.5 to 2.5 wt%

Chromium (Cr) is an element that improves hardenability and increases the strength of steel. When the Cr content is less than 0.5%, the above-mentioned effect is insufficient and the desired strength can not be secured. On the other hand, when the Cr content exceeds 2.5%, there is a problem that the ductility of the steel sheet deteriorates. Therefore, the Cr content is preferably 0.5 to 2.5%.

Al: not more than 0.05% by weight

Aluminum (Al) is concentrated on the surface of the steel sheet to deteriorate the plating ability, while suppressing carbide formation, thereby increasing the ductility of the steel. On the other hand, in the case of the thin slab, the reheating process in the conventional hot melt mill can be omitted, energy can be saved and the production can be improved, but the temperature of the slab surface or the edge portion may be lowered due to cooling of the surface of the slab. As a result, the AlN is excessively precipitated and deterioration of the high temperature ductility may deteriorate the edge quality of the cast steel and / or the bar. Therefore, in the present invention, the Al content should be controlled as low as possible and preferably controlled to be not more than 0.05%.

Ti: 0.005 to 0.10 wt%

Titanium (Ti) is an element for forming precipitates and nitrides, which increases the strength of steel. In addition, Ti is an element that reduces the amount of AlN precipitate by removing solute N through the formation of TiN near the solidification temperature, thereby preventing deterioration of high temperature ductility and reducing sensitivity to edge cracking. When the Ti content is less than 0.005%, the above-mentioned effect is insufficient. On the other hand, if the Ti content exceeds 0.10%, the manufacturing cost may increase and the ductility of the ferrite may be deteriorated. Therefore, the Ti content is preferably 0.01 to 0.10%.

Nb: 0.005 to 0.10 wt%

Niobium (Nb) is an element effective for increasing the strength and grain size of a steel sheet. When the Nb content is less than 0.005%, the above-mentioned effect is insufficient. On the other hand, when the Nb content exceeds 0.10%, the manufacturing cost may increase, the ductility of the ferrite may decrease, and the edge crack of the slab / bar may be caused. Therefore, the content of Nb is preferably 0.005 to 0.10%.

B: 0.0005 to 0.004% by weight

Boron (B) is an element that serves to delay the transformation of austenite into pearlite during cooling during annealing. When the B content is less than 0.0005%, the above-mentioned effect is insufficient, and when the B content exceeds 0.004%, the curing ability is greatly increased and deterioration of workability may be caused. Therefore, the content of B is preferably 0.0005 to 0.004% by weight.

N: 0.001 to 0.010 wt%

Nitrogen (N) is an austenite stabilizing and nitriding element. When the N content is less than 0.001%, the above-mentioned effect is insufficient. On the other hand, when the N content exceeds 0.010%, the precipitation strengthening effect is increased by reacting with the precipitate-forming element, but this may cause a drastic decrease in ductility. Therefore, the N content is preferably 0.001 to 0.010%.

On the other hand, the alloy composition of the cold-rolled steel sheet of the present invention preferably satisfies the following relational expressions (1) to (4). [Cp] in the following relational expressions 1 to 4 is 0.09-0.038C-0.013Mn-0.028Si + 0.00015Cr, and [Ceq] is C + Si / 30 + Mn / 20 + 2P + 3S.

[Relation 1] [C]? [Cp]

In the above formula 1, [Cp] is a formula for obtaining the critical C content at which the apodization reaction occurs. When the C content of the low-carbon content exceeds [Cp], an apodization reaction occurs, So that it is possible to cause an accident due to the leakage of molten steel. Therefore, it is preferable that the C content of molten steel is equal to or lower than [Cp] in order to produce a sound thin slab through high-speed casting.

[Relation 2] 1.5? [C 占 Cp] 占 1000? 3.5

When the value of [C x Cp] x 1000 is less than 1.50 in the above-mentioned relational expression 2, it may be difficult to obtain the desired strength. When the value exceeds 3.5, the epoxidation reaction may occur, .

[Relation 3] 165? [Ceq] x 1000? 235

[Ceq] x1000 is a component relational expression for securing the weldability of the steel sheet and the mechanical properties of the welded portion. When [Ceq] x1000 is less than 165, the curing ability is low and it is difficult to secure the desired tensile strength. The mechanical properties of the welded portion may be deteriorated.

[Relation 4] 50? [Ceq] / ([CxCp])? 150

If [Ceq] / ([CxCp]) is less than 50 in the above-mentioned relational expression 4, apodization reaction may occur and it may be difficult to manufacture sound thin slabs. If it exceeds 150, the mechanical properties of the welds may be deteriorated have.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

On the other hand, the cold-rolled steel sheet of the present invention may contain at least one selected from the group consisting of V, Mo, Cu, Ni, Zn, Se, Sb, Zr, W, Ga, By weight is not more than 0.2% by weight. The tramp element is an impurity element derived from scrap used as a raw material in the steelmaking process, ladle or tundish refractory, etc. When the total amount exceeds 0.2%, the surface / edge crack of the thin slab and the cold- It is possible to reduce the surface quality of the substrate.

The cold-rolled steel sheet of the present invention preferably has a microstructure in an area fraction of 30 to 70% of ferrite and 30 to 70% of martensite and autotempered martensite sum. When the ferrite fraction is more than 70%, it is difficult to secure the desired tensile strength. When the ferrite fraction is less than 30%, the fraction of the remaining martensite and autotempered martensite structure becomes too high to secure the elongation and stretch flangeability. . When the sum of the martensite and the auto-tempered martensite fraction exceeds 70%, the strength becomes too high to secure workability. If the sum is less than 30%, it may be difficult to secure the desired strength. Auto-tempered martensite refers to a structure having properties similar to those of tempered martensite without any additional tempering treatment. The auto-tempered martensite has a martensitic structure in which C is supersaturated at a certain temperature or higher, and carbides [M = Fe, Cr, Mn, etc.) C ₃ ] to ensure ductility.

At this time, it is preferable that the average short axis size of the ferrite crystal grains is 0.2 to 3.0 mu m. The smaller grain size is advantageous in terms of strength and workability, but it is difficult to control the grain size to 0.2 탆 or less in the rolling and annealing conditions of the present invention. If the average short axis grain size of the ferrite exceeds 3.0 탆, it may be difficult to secure the desired strength and workability. It is more preferable that the average short axis size of the ferrite crystal grains is in the range of 0.2 to 3.0 mu m.

The average grain size of the martensite and the auto-tempered martensite is preferably 0.2 to 3.0 mu m. In the present invention, the martensite and the auto-tempered martensite structure are important for ensuring the desired strength. The smaller the grain size is, the more advantageous in securing strength and workability. However, in the rolling and annealing conditions of the present invention, it is difficult to control the average minor axis grain size of martensite and autotempered martensite to 0.2 탆 or less, If it exceeds 3.0 탆, it may be difficult to secure the desired strength and workability. The average short axis size of the crystal grains of the martensite and the auto-tempered martensite is more preferably in the range of 0.2 to 3.0 mu m.

The average short axis size of the martensite and the auto-tempered martensite is preferably 100 nm or less. When the average short axis size of the laths of the martensite and the auto tempered martensite exceeds 100 nm, it may be difficult to secure the desired strength. In the present invention, the smaller the average shortening size of the lath is, the more advantageous in securing the strength, so that the lower limit is not set.

Meanwhile, the cold-rolled steel sheet of the present invention preferably contains precipitates of M (X) (M = Nb, Ti, X = C, N) having an average size of 1 to 30 nm. (TiNb) C, (TiNb) N, (TiNb) (C, N) NbC, NbN, Nb (C, N), TiC, TiN, Ti N), and (NbTi) C, (NbTi) N, (NbTi) (C, N) and the complex precipitates thereof having a high content of Nb. If the size of the precipitate exceeds 30 nm, it may be difficult to effectively secure the strength, and it may be difficult to control to 1 nm or less under the rolling and annealing conditions of the present invention. Therefore, the average size of the M (X) , And more preferably in the range of 1 to 15 nm.

The average distance between the M (X) (M = Nb, Ti, X = C, N) precipitates is preferably 10 to 150 nm. If it is more than 150 nm, it may be difficult to secure a desired strength, and the grain refining effect may be insignificant. On the other hand, when the thickness is less than 10 nm, the number of precipitates is too much and the strength becomes high, which may be difficult to secure workability. Therefore, the average interval between the M (X) precipitates is preferably 10 to 150 nm.

In the cold-rolled steel sheet provided by the present invention, the cold-rolled steel sheet may have a tensile strength of 800 MPa or more, an elongation of 10% or more, a strip length and width tensile strength deviation of 35 MPa or less, and a stretch flange ratio of 35% or more. Further, the cold-rolled steel sheet of the present invention can have a thickness of 1 mm or less, and a high water-rejection ratio can be secured.

In addition, the cold-rolled steel sheet of the present invention preferably has a ductility ratio (CTS / TSS x 100) of 35% or more of an electrical resistance spot weld. The CTS is the Cross Tensile Strength (kN), and TSS is the Tensile Shear Strength (kN). The ductility ratio is defined as the ratio of the TSS to the CTS and is used as a comprehensive index to judge the mechanical properties of the weld in the electrical resistance spot welding of the Advanced High Strength Steel (AHSS). The cold-rolled steel sheet of the present invention may have a ductility ratio of 35% or more at an appropriate welding current range capable of ensuring a sound weld. If it is less than 35%, the strength of the welded portion and the collision stability are low, so that the passenger can not be stably protected in the event of a body collision. On the other hand, the appropriate welding current range is defined as the range between the lower limit current satisfying the minimum nugget diameter of 4t ^1/2 (t = thickness of material) and the upper limit current immediately before the expulsion phenomenon in which the liquid phase in the molten portion protrudes out of the weld it means.

Hereinafter, the cold-rolled steel sheet manufacturing method of the present invention will be described.

FIG. 1 is a schematic diagram of a facility for a performance-to-rolling direct process that can be applied to the present invention, and is a schematic diagram of a performance-to-rolling direct process facility applicable to the manufacture of hot rolled steel sheets for obtaining cold- rolled steel sheets. A cold-rolled steel sheet having a small material deviation and excellent stretch flangeability and an excellent water-repelling ratio according to an embodiment of the present invention can be manufactured from a hot-rolled steel sheet produced by applying a performance-to-rolling direct connection facility as shown in Fig. The rolling-to-rolling direct-connection facility comprises the steps of: preparing a thin slab a of a first thickness in a continuous casting machine 100 and forming the slab in a roughing mill 400 to produce a bar b of a second thickness thinner than the first thickness And a hot rolled steel sheet (c) having a second thickness and a third thickness is produced in the finishing mill (600). The hot-rolled steel sheet (b) having a thickness of 3.0 mm or less can be continuously rolled because there is no coil box between the rough rolling mill (400) and the finish rolling mill (600) It is possible to produce it stably. Surface scaling is easy due to the finishing mill scale breaker (FSB) 500 in front of the roughing mill scale breaker (RSB) and the finish rolling mill 600 before the roughing mill 400 It is possible to produce cold rolled steel sheets having excellent surface quality during pickling and cold rolling of hot rolled steel sheets in a subsequent process. Further, it is possible to perform isothermal / constant-speed rolling at a rolling speed difference of 10% or less between the top and tail in one strip in the finish rolling step, so that the strip width and the longitudinal direction temperature deviation are remarkably low, Out Table, ROT), it is possible to manufacture steel sheet with excellent material deviation. The thus rolled strip is cut by a high-speed shear machine 800 and wound by a winder 900. Meanwhile, the finishing rolling scale breaker 500 may be provided with a heater 200 for further heating the bar.

2 is another schematic diagram of a facility for a performance-rolling direct process that can be applied to the present invention. The apparatus for direct rolling-to-rolling process disclosed in FIG. 2 is substantially identical in construction to the apparatus disclosed in FIG. 1, but includes a heater 200 'for further heating a slab in front of the rough rolling mill 400, It is possible to lower the occurrence of edge defects and is advantageous in securing the surface quality. In addition, a space of at least one slab length is secured before the roughing mill, and batch rolling is possible.

The high-strength cold-rolled steel sheet of which the material deviation of the present invention is small, and excellent in the stretch flangeability and the rate of realization can be produced in all of the performance-rolling direct connection facilities disclosed in Figs.

Hereinafter, one embodiment of a method of manufacturing a hot-rolled steel sheet applicable to production of a cold-rolled steel sheet of the present invention will be described in detail.

First, molten steel having the above-described alloy composition is continuously cast to obtain a thin slab. At this time, the continuous casting is preferably performed at a casting speed of 4.0 to 8.0 mpm (m / min). The reason why the casting speed is set to 4.0 mpm or more is that a high speed casting and rolling process are connected and a casting speed higher than a certain level is required to secure the target rolling temperature. However, if the casting speed is slow, there is a risk of segregation from the cast steel. If such a segregation occurs, it is difficult to secure strength and workability, and the risk of material variation in the width direction or the longitudinal direction is increased. If it exceeds 8.0 mpm, the operation success rate may be reduced due to instability of the molten steel bath surface. Therefore, the casting speed is preferably in the range of 4.0 to 8.0 mpm.

At this time, the thin slab preferably has a thickness of 60 to 120 mm. When the thickness of the thin slab is more than 120 mm, high-speed casting is difficult, and the rolling load during rough rolling is increased. When the thickness is less than 60 mm, the temperature of the cast steel is rapidly decreased and uniform texture is hardly formed. In order to solve this problem, it is possible to additionally provide a heating apparatus, but this is a factor for improving the production cost, so it is preferable to exclude it. Therefore, the thickness of the thin slab is preferably in the range of 60 to 120 mm.

After the step of obtaining the thin slab, cooling water may be sprayed to the thin slab to remove the scale. For example, cooling water of 50 ° C or less is sprayed at a pressure of 50 to 350 bar from a roughing scale breaker (RSB) nozzle to remove the surface scale of the thin slab to a thickness of 250 μm or less . If the pressure is less than 50 bar, a large amount of arithmetic scale scale may remain on the thin slab surface and the surface quality may become dull after pickling. On the other hand, if the temperature exceeds 350 bar, the edge temperature may drop sharply and an edge crack may occur.

Thereafter, the thin slab is roughly rolled to obtain a bar. For example, a continuously cast thin slab can be rough rolled in a roughing mill consisting of 2 to 5 stands.

On the other hand, the surface temperature of the thin slab drawn in the rough rolling is preferably 900 to 1200 ° C. If the surface temperature of the thin slab is less than 900 ° C, there is a possibility that cracks may occur in the edge of the bar during the rough rolling load increase and rough rolling, which may lead to defects at the edge of the hot-rolled steel sheet. If the surface temperature of the thin slab is more than 1200 ° C, problems such as deterioration of the hot-rolled surface due to the remnant of the hot-rolled scale may occur. In addition, the internal temperature of the cast steel may be too high to cause the non-solidification, so that casting may be interrupted due to swelling of the cast steel before rough rolling. In addition, bulging may occur and mold level hunting (MLH) may occur severely, thereby making it difficult to decelerate at a peripheral speed and cast at a high speed. In other words, the molten steel in the mold may be severely shaken, and high-speed casting may be difficult. In order to instantaneously stabilize the casting operation, the peripheral speed must be reduced. However, surface quality and strength can not be ensured and continuous continuous rolling may be difficult .

Also, immediately after the rough rolling, the edge temperature of the bar is preferably 800 to 1100 ° C. When the edge temperature of the bar is less than 800 ° C, a large amount of complex precipitates such as NbC, Nb (C, N), (Nb, Ti) (C, N), AlN and BN are produced in the edge portion of the bar, There is a problem that the sensitivity to edge crack generation becomes very high as the surface roughness decreases. On the other hand, when the edge portion temperature is higher than 1100 ° C, the temperature of the central portion of the thin slab becomes too high, so that a large number of arithmetic scale may occur and the surface quality after the pickling may become poor.

And the step of removing the scale by spraying the cooling water to the bar after the step of obtaining the bar. 3 is a schematic diagram showing one embodiment of a Finishing Mill Scale Breaker (FSB). As shown in FIG. 3, a Finishing Mill Scale Breaker (hereinafter referred to as FSB) 500 includes a cooling water spray nozzle 502. The cooling water 504 is injected from the cooling water injection nozzle 502 to remove the scale formed on the surface of the bar. In the step of spraying the cooling water to the bar to remove the scale, it is preferable to spray the cooling water of 50 ° C or less at a pressure of 50 to 350 bar to remove the scale to a thickness of 20 μm or less. If the pressure is less than 50 bar, scale removal is insufficient, so that a large scale of scale-like (scaly) scale is formed on the surface of the steel sheet after finishing rolling, resulting in poor surface quality after pickling. On the other hand, when the pressure is higher than 350 bar, the finish rolling temperature is lowered to a temperature lower than Ar3, and the rolling load is rapidly increased, resulting in poor ducting, resulting in a shutdown of operation.

In the present invention, the overlap area ratio of the cooling water 502 injected from the cooling water injection nozzle 502 of the finishing rolling scale breaker 500 preferably satisfies the following relational expression (5). Here, the overlap area ratio of the cooling water 502 means the ratio of the overlap area A of the cooling water to the total area of the cooling water sprayed on the bar surface. Although the combined area ratio of the cooling water influences the scale removal side, it also influences the temperature in the width direction of the steel sheet, which may cause a material deviation, so that it is preferable to control it appropriately. If (X ³ -85X ² + 1800X) / 1000 is less than 5 in the following relation (5), a temperature deviation in the width direction may occur and a material deviation may occur severely.

[Relation 5] (X ³ -85X ² + 1800X) / 1000? 5

 (In the above-mentioned relational expression 5, X means the overlapping area ratio (%) of the cooling water sprayed to the steel sheet from the finish rolling scale breaker.)

Thereafter, the above bars are subjected to hot rolling at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet. During the finish hot rolling, finishing rolling can be performed in a finishing mill having, for example, 3 to 6 stands. Particularly, in order to minimize the longitudinal material deviation of the strip, it is important to control the temperature deviation in the last rolling mill to be 75 DEG C or less during the manufacture of one strip. On the other hand, the temperature deviation means a difference between the maximum value and the minimum value of the rolling temperature in the last rolling mill.

When the finish rolling temperature is lower than Ar3, the load of the roll during hot rolling is greatly increased to increase the energy consumption and the operation speed, and a sufficient austenite fraction can not be secured, and the target microstructure and material can not be secured. Also, when the temperature of the inlet side of the last rolling mill exceeds 75 캜 during the manufacturing of one strip in the last rolling mill, the austenite and ferrite fraction may be significantly deviated in the width direction, and the material deviation may be increased.

In addition, it is preferable that the speed deviation of the last mill during the production of one strip in the last mill is not more than 60 mpm. If the speed difference of the last rolling mill exceeds 60mpm, the temperature and the rolling load become uneven, resulting in material and thickness variations of the hot rolled steel sheet, and uneven rolling during cold rolling can increase the thickness variation of the final product. The speed deviation is more preferably 55 mpm or less, and more preferably 50 mpm or less. On the other hand, the speed deviation means the difference between the maximum value and the minimum value of the rolling speed in the last rolling mill.

On the other hand, the average rolling speed of the final rolling mill during the finish rolling is preferably 100 to 600 mpm. If the average passing speed of the final rolling mill is more than 600 mPm during the finishing rolling, there may occur an accident such as a plate rupture, and a uniform temperature may not be secured due to difficulty in isothermal and constant speed rolling. On the other hand, in the case of less than 100 mpm, the finishing rolling speed is too slow to secure the finishing rolling temperature.

On the other hand, the finishing rolling can be performed in a finishing mill made up of three to six stands of a bar made in a roughing mill.

The hot rolled steel sheet obtained as described above may have a thickness of 3.0 mm or less, more preferably 2.5 mm or less.

Thereafter, the hot-rolled steel sheet is wound at 400 to 650 ° C. When the coiling temperature is lower than 400 캜, the martensite transformation is promoted and the strength becomes too high, so that there may be a problem that the rolling property and the shape may be poor when cold rolling. If it exceeds 650 캜, The surface quality may be deteriorated. Therefore, it is preferable to control the coiling temperature to 400 to 650 占 폚.

On the other hand, the above-described method for producing a hot-rolled steel sheet is characterized in that the above-described respective steps are performed continuously by using the continuous rolling mode in the performance-to-rolling direct connection process.

Thereafter, the rolled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio in the cold rolling is preferably in the range of 40 to 70%. If the reduction rate is less than 40%, there is a risk that recrystallization does not occur during annealing. If the reduction rate exceeds 70%, the rolling deformation resistance increases greatly and rolling becomes difficult, so that the reduction rate is preferably in the range of 40 to 70% . It is preferable that the cold rolled steel sheet has a widthwise average steepness of 1.0% or less. If the widthwise average steepness exceeds 1.0%, the shape of the cold-rolled steel sheet is poor and it is difficult to pass through the annealing line, which may make production of the final product difficult. Here, the steepness is an important index for evaluating the shape of the strip by dividing the wave width (wave height) of the cold rolled steel strip by the length (wave length) of the wave and multiplying by 100 (wave height / wavelength × 100).

On the other hand, before the cold rolling, a step of pickling the hot-rolled steel sheet to remove the oxide layer may be further included.

After the cold rolling, the cold-rolled steel sheet is continuously annealed in a temperature range of Ac3-50 ° C to Ac3 + 30 ° C. The present invention is for producing a high-strength steel sheet excellent in both tensile properties and elongation flangeability. In order to obtain such a steel sheet, control of the subsequent annealing step is important. Particularly, in order to obtain ferrite and martensite and auto-tempered martensite structure as a final microstructure, and to obtain intended mechanical properties from these, the temperature range during final annealing is very important. When the annealing temperature is lower than Ac3-50 deg. C, strength decreases due to a large amount of residual ferrite. When the annealing temperature exceeds Ac3 + 30 deg. C, the austenite fraction is too high, It may be difficult to secure ductility and increase surface oxides such as Si and Mn. Therefore, it is preferable that the annealing temperature has a range of Ac3-50 ° C to Ac3 + 30 ° C. On the other hand, the above-mentioned Ac3 means a temperature at which ferrite starts to be transformed into austenite at the time of heating.

The continuously annealed cold rolled steel sheet is first cooled to a temperature range of Ar1 to Ar3 at an average cooling rate of 1 to 10 占 폚 / s. If the primary cooling end temperature is lower than Ar1, a large amount of ferrite transformation may occur and the strength of the steel may be lowered. On the other hand, if the primary cooling end temperature is higher than Ar3, the austenite fraction is too high, And the final cooling section is too long, so that the quality of the shape of the steel sheet may be deteriorated. The above-mentioned Ar3 means a temperature at which a transformation of austenite into ferrite starts at the time of cooling, and Ar1 means a temperature at which a complete transformation of austenite into ferrite is completed upon cooling. On the other hand, the cooling stop temperature is more preferably in the range of Ar 1 + 50 ° C to Ar 3 - 20 ° C.

The primary cooling rate is preferably in the range of 1 to 10 占 폚 / s. When the primary cooling rate is less than 1 캜 / s, a large amount of microstructures such as ferrite and pearlite are formed, and it may be difficult to secure the desired strength in the present invention. On the other hand, when the primary cooling rate exceeds 10 DEG C / s, abrupt bainite transformation or martensitic transformation occurs, which may cause a problem in that the quality of the steel sheet may deteriorate and the target ferrite fraction can not be secured It may be difficult to ensure ductility by increasing the hard tissue fraction in the final cooling, and it is preferable that the primary cooling rate is in the range of 1 to 10 ° C / s.

Thereafter, the primary cooled cold rolled steel sheet is secondarily cooled to a temperature range of Mf-50 ° C to Ms + 50 ° C at an average cooling rate of 5 to 30 ° C / s. When the secondary cooling quench temperature is lower than Mf-50 ° C, the supersaturated C in the martensite is hardly precipitated as a carbide, so that the transformation into the martensitic structure having a better ductility than that of the martensitic structure occurs little and the ductility and stretch flangeability are secured When the temperature exceeds Ms + 50 占 폚, it is difficult to obtain martensite and the target strength may not be obtained. The secondary cooling stop temperature is in the range of Mf-50 占 폚 to Ms + 50 占 폚 . On the other hand, the above-mentioned Ms is the temperature at which the martensitic transformation begins, and Mf is the temperature at which the martensitic transformation is completed.

The secondary cooling rate is preferably in the range of 5 to 30 DEG C / s. When the secondary cooling rate is less than 5 占 폚 / s, it is difficult to ensure a sufficient martensite fraction and it is difficult to secure a desired strength. When the secondary cooling rate exceeds 50 占 폚 / s, It may be difficult to secure ductility and stretch flangeability because the martensite fraction is exceeded, and the secondary cooling rate is preferably in the range of 5 to 50 ° C / s.

Hereinafter, the present invention will be described in more detail with reference to Examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example 1)

The molten steel having the alloy composition shown in Tables 1 and 2 was prepared, and then the molten steel was continuously cast at a casting speed of 6 mpm in a continuous continuous rolling mode by applying a performance-rolling direct process, thereby obtaining a thin slab having a thickness of 90 mm, Hot-rolled steel sheets having a thickness of 2.4 mm were produced under the manufacturing conditions shown in Table 3 below. However, in the case of the conventional example, a slab having a thickness of 250 mm was cast from a conventional hot-rolling mill, and then a hot-rolled steel sheet having a thickness of 3.5 mm was produced in the conventional batch process under the manufacturing conditions shown in Table 3 below. Then, each hot-rolled steel sheet thus produced was pickled and cold-rolled under the conditions shown in Table 4 to obtain a cold-rolled steel sheet (Full Hard material, hereinafter referred to as FH material), followed by annealing, primary and secondary cooling, (Cold rolled material, hereinafter referred to as CR material). The temperatures Ac3, Ar3, Ar1, Ms, and Mf in Table 3 were calculated using JmatPro-v9.1, a commercial thermodynamic software.

The cold-rolled steel sheets thus produced were measured for their microstructure and mechanical properties, and the results are shown in Table 6 below. The shape was evaluated by the steepness (= wave height / wavelength X100) obtained by dividing the wave width (wave height) of the cold-rolled steel sheet by the wave length (wavelength) and multiplying by 100.

The area fraction of ferrite (F), martensite (M) and auto-tempered martensite (AM) was 10 times at 500 times and 5,000 times magnification using an optical microscope and a scanning electron microscope (SEM) Was randomly photographed, and the area ratio was measured using Image-Plus Pro software, and the result was expressed as an average value.

The uniaxial grain size and distribution of ferrite (F), martensite (M) and auto-tempered martensite (AM) were randomly photographed at a magnification of 3,000 times using SEM at 10 locations, And the size of the short axis was measured and expressed as an average value.

The tensile strength is measured by taking JIS No. 5 specimens in a direction perpendicular to the rolling direction (C direction) at a width of w / 2. The length deviation of the tensile strength is measured by measuring the tensile strength And the difference in intensity values. The Hole Expanding Ratio (HER) was measured by punching the hole with a diameter of 10.8 mm and then pushing it up to the cone to increase the diameter of the expanded hole until the crack was generated in the circumference portion to the initial diameter (10.8 mm) Calculated as a percentage.

The ductility ratios were evaluated by welding using the electric resistance spot welding conditions (specified in ISO 18728-2) listed in Table 5. The results are shown in Table 6.

division
Alloy composition (% by weight) C Mn Si P S Al Cr Ti Nb B N Inventive Steel 1 0.048 2.56 0.12 0.007 0.001 0.025 0.96 0.045 0.031 0.0023 0.0062 Invention river 2 0.041 2.25 0.21 0.015 0.0014 0.026 1.2 0.047 0.028 0.0025 0.0059 Invention steel 3 0.048 2.61 0.15 0.008 0.0015 0.035 0.8 0.05 0.03 0.0025 0.0035 Comparative River 1 0.065 1.95 0.15 0.008 0.001 0.035 1.03 0.029 0.032 0.0023 0.0051 Inventive Steel 4 0.035 2.95 0.1 0.008 0.0011 0.04 1.5 0.031 0.035 0.0035 0.0045 Comparative River 2 0.095 1.85 0.45 0.01 0.001 0.025 1.3 0.045 0.031 0.0023 0.0062 Comparative Steel 3 0.022 1.85 0.35 0.007 0.001 0.031 2 0.041 0.035 0.0025 0.0058 Invention steel 5 0.041 2.35 0.12 0.01 0.0012 0.032 0.56 0.045 0.031 0.0025 0.005 Invention steel 6 0.045 2.55 0.1 0.026 0.0016 0.035 One 0.039 0.029 0.0026 0.0055 Comparative Steel 4 0.032 2.85 0.48 0.022 0.004 0.029 0.95 0.045 0.035 0.0025 0.0059 Invention steel 7 0.054 2.25 0.15 0.007 0.001 0.031 1.09 0.039 0.039 0.0024 0.0045 Comparative Steel 5 0.024 2.01 0.11 0.05 0.0033 0.035 1.1 0.041 0.031 0.0023 0.0041 Conventional steel 0.049 2.53 0.15 0.02 0.003 0.035 1.01 0.05 0.03 0.0025 0.0035

division [Cp] Relation 1 Satisfaction Relation 2 [Ceq] Relation 3 Relation 4 Inventive Steel 1 0.058 ○ 2.48 0.197 197 79 Invention river 2 0.053 ○ 2.19 0.195 195 89 Invention steel 3 0.05 ○ 2.41 0.204 204 85 Comparative River 1 0.058 × 3.78 0.187 187 49 Inventive Steel 4 0.048 ○ 1.67 0.205 205 123 Comparative River 2 0.05 × 4.74 0.226 226 48 Comparative Steel 3 0.046 ○ 1.22 0.143 143 117 Invention steel 5 0.055 ○ 2.24 0.186 186 83 Invention steel 6 0.052 ○ 2.36 0.233 233 98 Comparative Steel 4 0.038 ○ 1.23 0.247 247 200 Invention steel 7 0.055 ○ 2.95 0.189 189 64 Comparative Steel 5 0.06 ○ 1.44 0.238 238 165 Conventional steel 0.051 ○ 2.51 0.230 230 83 [Cp] is 0.09-0.038C-0.013Mn-0.028Si + 0.00015Cr, and [Ceq] is C + Si / 30 + Mn / 20 + 2P + 3S.

division
Grade Nr.
Hot-rolled steel sheet thickness (mm)
Finishing The last rolling mill Coiling temperature
(° C) Shipping speed
(mpm) Speed deviation
(mpm) Ar3
(° C) Rolling temperature
(° C) Temperature range
(° C) Inventory 1 Inventive Steel 1 2.4 225 35 750 781 45 550 Inventory 2 Invention river 2 2.4 225 40 760 778 44 546 Inventory 3 Invention steel 3 2.4 225 38 751 779 40 551 Comparative Example 1 Comparative River 1 Stop casting of molten steel Honorable 4 Inventive Steel 4 2.4 225 45 725 782 50 551 Comparative Example 2 Comparative River 2 Stop casting of molten steel Comparative Example 3 Comparative Steel 3 2.4 225 38 785 779 50 556 Inventory 5 Invention steel 5 2.4 225 25 780 781 48 550 Inventory 6 Invention steel 6 2.4 225 35 770 780 51 549 Comparative Example 4 Comparative Steel 4 2.4 225 40 790 776 48 551 Honorable 7 Invention steel 7 2.4 225 42 800 785 50 549 Comparative Example 5 Comparative Steel 5 2.4 225 43 20 780 52 550 Conventional Example 1 Conventional steel 3.5 650 250 747 935 135 553

division Grade Nr. Cold rolled steel plate thickness (mm) Cold
Reduction rate
(%) Ac3
(° C) Ar3
(° C) Ar1
(° C) Ms
(° C) Mf
(° C) Annealing
Temperature
(° C) Primary cooling Secondary cooling Stop temperature
(° C) Cooling
speed
(° C / s) Stop temperature
(° C) Cooling
speed
(° C / s) Inventory 1 Inventive Steel 1 1.2 57 802 750 515 384 272 800 651 5 373 15 Inventory 2 Invention river 2 1.2 57 816 760 560 396 285 815 652 5 371 15 Inventory 3 Invention steel 3 1.2 57 805 751 505 387 275 800 655 5 379 15 Comparative Example 1 Comparative River 1 Stop casting of molten steel Honorable 4 Inventive Steel 4 1.2 57 789 725 540 364 250 786 645 5 374 15 Comparative Example 2 Comparative River 2 Stop casting of molten steel Comparative Example 3 Comparative Steel 3 1.2 57 829 785 600 404 293 825 649 5 370 15 Inventory 5 Invention steel 5 1.2 57 817 780 550 407 296 814 651 5 371 15 Inventory 6 Invention steel 6 1.2 57 810 770 515 376 263 805 655 5 370 15 Comparative Example 4 Comparative Steel 4 1.2 57 807 790 410 370 256 798 652 5 371 15 Honorable 7 Invention steel 7 1.2 57 810 800 555 391 280 805 658 5 369 15 Comparative Example 5 Comparative Steel 5 1.2 57 840 820 580 416 306 831 657 5 371 15 Conventional Example 1 Conventional steel 1.5 57 801 747 505 385 272 794 645 5 320 15

Welding conditions (Single phase AC, 60Hz) Electrode Specifications Pressing force (kN) Welding time (cycle) Holding time (cycle) Squeeze time (cycle) Cooling water quantity
(l / min) Welding current (kA) 4 17 17 40 4 7 Cr-Cu alloy,
R type, 6mm

division
Width direction FH material
Average
Steepness degree
(%) strip
Length
direction
Cutting
Length
(m) Microstructure fraction (%) Crystal grain
size
(탆) surrender
burglar
(MPa) Seal
burglar
(MPa) Elongation
(%) Lengthwise
The tensile strength
Deviation
(? MPa) HER
(%) Ductility ratio
(%) F M + A.M F M + A.M Inventory 1 0.33 22 55 45 1.35 1.05 655 835 15 28 57 55 Inventory 2 0.29 31 58 42 1.35 1.10 638 825 16 26 58 57 Inventory 3 0.35 21 55 45 1.29 1.05 662 840 15 29 56 56 Comparative Example 1 Stop casting of molten steel Honorable 4 0.30 22 56 44 1.31 1.11 668 839 15 29 54 56 Comparative Example 2 Stop casting of molten steel Comparative Example 3 0.32 28 72 28 2.29 1.20 615 790 21 21 60 60 Inventory 5 0.35 23 59 41 1.28 1.15 635 820 17 20 58 58 Inventory 6 0.85 21 28 62 1.25 1.20 692 900 12 22 42 38 Comparative Example 4 0.97 22 25 65 1.25 1.21 699 920 11 21 40 30 Honorable 7 0.41 20 59 41 1.32 1.10 654 820 16 28 60 53 Comparative Example 5 0.54 23 73 27 2.25 1.12 624 795 20 29 62 32 Conventional Example 1 0.94 140 45 55 1.56 1.15 644 841 15 55 52 50 F is ferrite, M is martensite, and A.M is auto-tempered martensite.

As can be seen from Tables 1 to 6, in the case of the alloying composition, the constitutional relational expressions 1 to 4 proposed by the present invention, and the inventive examples 1 to 7 satisfying the manufacturing conditions, the target microstructure characteristics The tensile strength of not less than 800 MPa, the elongation of not less than 10%, the tensile strength in the longitudinal direction of not more than 35 MPa, the elongation flange ratio of not less than 35%, and the ductility ratio of not less than 35% Able to know. In particular, Inventive Examples 1 to 7 show that the strip cutting length is lower than that of Conventional Example 1 produced in the batch mode in the conventional hot melt mill, resulting in a high rate of yield and a small deviation in tensile strength in the longitudinal direction.

On the other hand, Comparative Examples 1 to 5 do not satisfy at least one of the alloy composition and the component relational expressions 1 to 4 proposed by the present invention, so that casting due to the molten steel outflow has occurred or due to a difference in the microstructure fraction, It can be seen that the strength and ductility ratio are not ensured.

4 and 5 are photographs of SEM micrographs and SEM micrographs of Inventive Example 1, respectively. As can be seen from FIGS. 4 and 5, ferrite (F) and auto-tempered martensite (AM) are composed of major phases and some martensite (M) structure is not tempered can confirm.

6 is a TEM photograph of a transmission electron microscope (TEM) of Inventive Example 1, which is a photograph of a magnification of 100,000 magnification obtained by enlarging the [X] and [Y] portions on the left 50,000 times and the right side on the left photograph. As can be seen from the tissue photographs marked with [X], it can be seen that the martensitic structure of about 20 nm or less is very clearly observed. And, can be seen from the organization picture labeled with [Y] as [X] does not clear the class than the martensitic structure of the portion, 10nm fine carbides of (Fe, Mn, Cr) or less to be ₃ C is observed auto-tempered It can be seen that de martensite structure exists in the invention steel.

7 and 8 are graphs showing the ferrite (F) single crystal grain size distribution of Inventive Example 1 and the martensite (M) + autothermite martensite (A.M) single axis grain size distribution, respectively. 7 and 8, ferrite (F) and martensite (M) + auto-temperate martensite (AM) structures having a minor axis grain size of 0.2 to 5.0 μm are distributed in Inventive Steel 1, Is in the range of 0.2 to 3.0 占 퐉.

9 is a photograph of a precipitate of Inventive Example 1 taken by a transmission electron microscope (TEM). Here, the precipitate size was a sample prepared by the carbon repulping method, and 5 sheets of 50,000 times and 10 sheets of 100,000 times were taken by TEM at random, and the precipitate size was measured using Image-Plus Pro software. As can be seen from these results, it can be seen that precipitates having a high Ti content ( _TiNb ) (C, N) of 30 nm or less and Nb _Ti (C, N) precipitates having a high Nb content are uniformly distributed.

10 and 11 are graphs showing the distribution of the size of the precipitate of Inventive Example 1 and the spacing between the precipitates, respectively. As can be seen from FIGS. 10 and 11, precipitates having a size of 1 to 30 nm exist in Inventive Example 1, and precipitates having a size of 1 to 15 nm are mainly distributed. It can also be seen that the precipitates are present in a large amount in the intervals of 10 to 150 nm.

(Example 2)

After the molten steel having the alloy composition of Inventive Steel 1 of Table 1 and the conventional steel was prepared, the molten steel was continuously cast at a casting speed of 6.8 mpm by applying a performance-rolling direct process to obtain a 90 mm thick thin slab, Rolled steel sheets having a thickness of 16 mm, and then hot-rolled steel sheets were produced under the manufacturing conditions described in the following Table 7, and the cold-rolled steel sheets were produced under the production conditions described in Table 8 below. The thus-prepared cold-rolled steel sheet was measured for microstructure, mechanical properties and ductility ratio, and the results are shown in Table 9 below. On the other hand, the physical properties were measured under the same conditions as in Example 1.

division
Grade Nr.
Hot-rolled steel sheet thickness (mm)
Finishing The last rolling mill Coiling temperature
(° C) Shipping speed
(mpm) Speed deviation
(mpm) Ar3
(° C) Rolling temperature
(° C) Temperature range
(° C) Honors 8 Inventive Steel 1 2.8 195 30 745 779 52 555 Comparative Example 6 Inventive Steel 1 2.8 195 65 745 775 45 547 Comparative Example 7 Inventive Steel 1 2.8 195 80 745 781 46 552 Comparative Example 8 Inventive Steel 1 2.8 195 35 745 780 80 559 Comparative Example 9 Inventive Steel 1 2.8 195 35 745 785 105 539 Comparative Example 10 Inventive Steel 1 2.8 195 35 745 781 45 542 Comparative Example 11 Inventive Steel 1 2.8 195 35 745 779 45 556 Comparative Example 12 Inventive Steel 1 2.8 195 30 745 781 45 550 Comparative Example 13 Inventive Steel 1 2.8 195 35 745 780 45 549 Comparative Example 14 Inventive Steel 1 2.8 195 35 745 776 50 551 Comparative Example 15 Inventive Steel 1 2.8 195 30 745 785 45 549 Comparative Example 16 Inventive Steel 1 2.8 195 35 745 780 40 550 Comparative Example 17 Inventive Steel 1 2.8 195 35 745 781 45 548 Proposition 9 Inventive Steel 1 2.4 195 35 745 780 50 551 Inventory 10 Inventive Steel 1 1.8 195 35 745 758 45 550 Conventional Example 2 Conventional steel 3.5 650 250 750 935 130 550 Conventional Example 3 Conventional steel 2.8 650 250 750 935 140 552 Conventional Example 4 Conventional steel 1.9 650 250 750 924 135 553

division Grade Nr. Cold rolled steel plate thickness (mm) Cold
Reduction rate
(%) Ac3
(° C) Ar3
(° C) Ar1
(° C) Ms
(° C) Mf
(° C) Annealing
Temperature
(° C) Primary cooling Secondary cooling Stop temperature
(° C) Cooling
speed
(° C / s) Stop temperature
(° C) Cooling
speed
(° C / s) Honors 8 Inventive Steel 1 1.2 57 802 745 515 390 278 805 649 5 368 15 Comparative Example 6 Inventive Steel 1 1.2 57 802 745 515 390 278 795 651 5 371 15 Comparative Example 7 Inventive Steel 1 1.2 57 802 745 515 390 278 796 653 5 376 15 Comparative Example 8 Inventive Steel 1 1.2 57 802 745 515 390 278 805 649 5 370 15 Comparative Example 9 Inventive Steel 1 1.2 57 802 745 515 390 278 801 645 5 375 15 Comparative Example 10 Inventive Steel 1 1.2 57 802 745 515 390 278 835 650 5 375 15 Comparative Example 11 Inventive Steel 1 1.2 57 802 745 515 390 278 749 651 5 370 15 Comparative Example 12 Inventive Steel 1 1.2 57 802 745 515 390 278 786 750 5 371 15 Comparative Example 13 Inventive Steel 1 1.2 57 802 745 515 390 278 786 500 5 369 15 Comparative Example 14 Inventive Steel 1 1.2 57 802 745 515 390 278 810 650 5 450 15 Comparative Example 15 Inventive Steel 1 1.2 57 802 745 515 390 278 816 652 5 216 15 Comparative Example 16 Inventive Steel 1 1.2 57 802 745 515 390 278 790 651 5 375 50 Comparative Example 17 Inventive Steel 1 1.2 57 802 745 515 390 278 795 653 5 370 3 Proposition 9 Inventive Steel 1 One 57 802 745 515 390 278 791 651 5 379 15 Inventory 10 Inventive Steel 1 0.75 57 802 745 515 390 278 795 650 5 375 15 Conventional Example 2 Conventional steel 1.5 57 805 750 510 386 274 818 651 5 318 15 Conventional Example 3 Conventional steel 1.2 57 805 750 510 386 274 819 652 5 320 15 Conventional Example 4 Conventional steel 0.75 61 805 750 510 386 274 - - - - -

division Width direction
FH ash
Steepness degree
Average
(%) strip
Length
direction
Cutting
Length
(m) Microstructure fraction
(area%) Crystal grain
size
(탆) surrender
burglar
(MPa) Seal
burglar
(MPa) Elongation
(%) Lengthwise
The tensile strength
Deviation
(? MPa) HER
(%) Ductility ratio
(%) F M + A.M F M + A.M Honors 8 0.29 42 56 44 1.35 1.05 655 835 15 28 57 55 Comparative Example 6 0.31 70 56 44 1.32 1.15 649 837 14 42 55 54 Comparative Example 7 0.33 95 60 40 1.29 106 650 839 14 45 56 54 Comparative Example 8 0.34 45 58 42 1.32 1.12 651 834 14 43 54 52 Comparative Example 9 0.32 41 59 41 1.51 1.21 652 380 15 47 56 51 Comparative Example 10 0.31 44 25 75 1.52 3.25 886 988 8 35 33 43 Comparative Example 11 0.33 43 78 22 3.52 2.01 560 735 18 29 64 56 Comparative Example 12 0.35 45 25 75 1.52 3.21 895 1010 7 32 32 43 Comparative Example 13 0.32 48 84 16 3.56 1.95 515 652 21 28 77 58 Comparative Example 14 0.38 51 72 28 3.49 2.03 605 779 16 29 58 55 Comparative Example 15 0.27 50 26 74 1.52 3.26 883 976 9 34 34 45 Comparative Example 16 0.38 51 20 80 1.56 3.35 956 1053 6 36 30 42 Comparative Example 17 0.32 52 75 25 3.34 1.99 575 756 17 28 69 55 Proposition 9 0.35 54 56 44 1.30 1.01 645 844 14 28 55 54 Inventory 10 0.37 55 55 45 1.20 1.01 635 839 14 29 57 53 Conventional Example 2 0.35 120 45 55 1.56 1.15 644 874 12 50 53 54 Conventional Example 3 0.49 125 44 56 1.59 1.21 652 869 12 61 52 52 Conventional Example 4 1.56 180 Failure to anneal due to defective shape F is ferrite, M is martensite, and A.M is auto-tempered martensite.

As can be seen from Tables 7 to 9, in the case of Inventive Examples 8 to 10 which satisfy both the alloy composition and the manufacturing conditions proposed by the present invention, the target microstructure characteristics are all satisfied, Mechanical properties. Particularly, the inventive examples 8 to 10 show that the strip cutting length is low compared to the conventional examples 2 and 3 produced in the batch mode in the hot-melt mill, so that the yield rate is high and the tensile strength deviation in the longitudinal direction is small.

On the other hand, in Comparative Examples 6 and 7, it was found that the rolling speed deviation in the final rolling mill during finish rolling exceeded the condition of the present invention, and the tensile strength in the longitudinal direction of the strip was severely deviated.

In Comparative Examples 8 and 9, it can be seen that the rolling temperature deviation in the final rolling mill during finish rolling exceeded the conditions of the present invention, and the tensile strength in the longitudinal direction of the strip was severely deviated.

In Comparative Examples 10 and 11, the annealing temperature deviates from the conditions of the present invention, and the microstructure to be obtained by the present invention is not ensured. As a result, the mechanical properties are low.

In Comparative Examples 12 and 13, the quenching temperature in the primary cooling was outside the conditions of the present invention, and the microstructure desired to be obtained by the present invention was not secured, and the mechanical properties were low.

In Comparative Examples 14 and 15, the quenching temperature in the secondary cooling is outside the conditions of the present invention, and the microstructure desired to be obtained by the present invention is not secured, and the mechanical properties are low.

In Comparative Examples 164 and 17, the cooling rate in the secondary cooling was out of the conditions of the present invention, and the microstructure desired to be obtained by the present invention was not secured. As a result, the mechanical properties were low.

On the other hand, in the case of Conventional Example 4, it was impossible to perform annealing due to the defective shape.

(Example 3)

After preparing a molten steel having the alloy composition of Inventive Steel 2 in Table 1, the molten steel was continuously cast at a casting speed of 6.8 mpm by applying a performance-rolling direct process, thereby obtaining a 90 mm thick thin slab, Hot rolled steel sheets were produced under the manufacturing conditions described in the following Table 10, and then cold rolled and annealed under the same conditions as in Example 2 of Table 4 to prepare cold rolled steel sheets. The thus obtained hot-rolled steel sheet and cold-rolled steel sheet were measured for tensile strength in the width direction, and the results are shown in Table 11 below.

division Grade Nr. Hot rolling
Steel plate
thickness
(mm) Finishing rolling scale
Breaker pressure
(bar) Finishing rolling scale
Breaker cooling water
Injection overlap area ratio (%) Air cooling
time
(° C / s) Cooling
speed
(° C / s) Coiling
Temperature
(° C) Cold rolling
Steel plate
thickness
(mm) Exhibit 11 Invention river 2 2.8 235 13 4.0 115 555 1.2 Inventory 12 Invention river 2 2.8 240 15 4.2 120 547 1.2 Inventory 13 Invention river 2 2.8 225 18 3.9 125 552 1.2 Inventory 14 Invention river 2 2.8 230 20 4.5 105 559 1.2 Honorable Mention 15 Invention river 2 2.8 250 24 4.1 100 539 1.2 Comparative Example 18 Invention river 2 2.8 235 33 4.3 110 542 1.2 Inventory 16 Invention river 2 2.8 240 9 4.2 125 556 1.2 Inventory 17 Invention river 2 2.8 235 7 4.3 125 550 1.2 Comparative Example 19 Invention river 2 2.8 235 2 4.4 125 549 1.2

division Grade Nr. Relation 5 Last rolling mill
Strip width direction
Temperature deviation (℃) Hot-rolled steel sheet Cold rolled steel plate Tensile strength in the transverse direction
Deviation (MPa) Tensile strength in the transverse direction
Deviation (MPa) Exhibit 11 Invention river 2 11.2 16 36 22 Inventory 12 Invention river 2 11.3 18 37 23 Inventory 13 Invention river 2 10.7 19 39 24 Inventory 14 Invention river 2 10.0 22 42 25 Honorable Mention 15 Invention river 2 8.1 29 45 27 Comparative Example 18 Invention river 2 2.8 44 59 46 Inventory 16 Invention river 2 10.0 19 36 25 Inventory 17 Invention river 2 8.8 28 44 27 Comparative Example 19 Invention river 2 3.3 38 54 41 [Relation 5] (X ³ -85X ² + 1800X) / 1000? 5
In the above formula (5), X represents the overlap area ratio (%) of the cooling water sprayed to the steel sheet from the finish rolling scale breaker.

As can be seen from Tables 10 to 11, in Examples 11 to 17, which satisfy the manufacturing conditions proposed by the present invention, the target tensile strength deviations are satisfied, It can be seen that

However, in the case of Comparative Examples 18 and 19, since the value of the relational expression 5 is less than 5, it can be seen that the material deviation is severely caused by the occurrence of the temperature deviation in the width direction.

a: Slab b: Bar
c: hot rolled steel sheet A: overlapping area of cooling water sprayed on the bar
100: continuous casting machine 200, 200 ': heater
300: RSB (Roughing Mill Scale Breaker, rough rolling scale breaker)
400: rough rolling mill
500: FSB (Fishing Mill Scale Breaker, Finishing Rolled Scale Breaker)
502: Cooling water injection nozzle
504: Cooling water
600: finishing mill 700: run-out table
800: High speed shear machine 900: Winder

Claims (21)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.030 to 0.080% of C, 1.5 to 3.0% of Mn, 0.03 to 0.50% of Si, 0.001 to 0.045% of Si, 0.0005 to 0.0035% of S, % Of Ti, 0.005 to 0.10% of Ti, 0.005 to 0.10% of Nb, 0.0005 to 0.004% of N, 0.001 to 0.010 of N, the balance Fe and other unavoidable impurities,
Satisfy the following relational expressions (1) to (4)
The microstructure is a high strength cold rolled steel sheet having an area fraction of 30 to 70% of ferrite and 30 to 70% of martensite and autotempered martensite: 30 to 70% and having a small material deviation and excellent elongation flangeability and water content.
[Relation 1] [C]? [Cp]
[Relation 2] 1.5? [C 占 Cp] 占 1000? 3.5
[Relation 3] 165? [Ceq] x 1000? 235
[Relation 4] 50? [Ceq] / ([CxCp])? 150
(Cp) in the above relational expressions 1 to 4 is 0.09-0.038C-0.013Mn-0.028Si + 0.00015Cr, [Ceq] is C + Si / 30 + Mn / 20 + 2P + 3S, , And the contents of Cr, P, and S respectively represent% by weight.)
The method according to claim 1,
The cold-rolled steel sheet has a material deviation of less than 0.2 wt% in total of at least one selected from the group consisting of V, Mo, Cu, Ni, Zn, Se, Sb, Zr, W, Ga, , High-strength cold-rolled steel sheet excellent in elongation flangeability and water content.
The method according to claim 1,
Wherein the ferrite crystal grains have a small material deviation with an average short axis size of 0.2 to 3.0 占 퐉 and excellent stretch flangeability and a real water rate.
The method according to claim 1,
Wherein the martensite and autotempered martensite grain grains have a small material deviation with an average short axis size of 0.2 to 3.0 占 퐉 and excellent stretch flangeability and yield.
The method according to claim 1,
A high-strength cold-rolled steel sheet excellent in stretch flangeability and water-repellency with less material deviation having an average short axis size of martensite and autotempered martensite less than 100 nm.
The method according to claim 1,
The cold-rolled steel sheet has a small material deviation including an M (X) (M = Nb, Ti, X = C, N) precipitate having an average size of 1 to 30 nm and is excellent in stretch flangeability and water content.
The method according to claim 1,
Wherein the average interval between the M (X) (M = Nb, Ti, X = C, N) precipitates is 10 to 150 nm, the material deviation is small, and the stretch flangeability and the water content are excellent.
The method according to claim 1,
The cold-rolled steel sheet has a tensile strength of 800 MPa or more, an elongation of 10% or more, a strip length and width tensile strength variation of 35 MPa or less, a material variation of 35% or more in elongation flange ratio, Excellent high strength cold rolled steel sheet.
The method according to claim 1,
The cold-rolled steel sheet has a thickness of 1 mm or less, less material deviation, and excellent stretch flangeability and water-rejection ratio.
The method according to claim 1,
The cold-rolled steel sheet has a low material deviation of 35% or more in the ductility ratio (CTS / TSS x 100) of the electrical resistance spot weld, and is excellent in elongation flangeability and water content.
(CTS is Cross Tensile Strength, kN), TSS is Tensile Shear Strength (kN).
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.030 to 0.080% of C, 1.5 to 3.0% of Mn, 0.03 to 0.50% of Si, 0.001 to 0.045% of Si, 0.0005 to 0.0035% of S, 0.001 to 0.10% of Ti, 0.005 to 0.10% of Nb, 0.0005 to 0.004% of N, 0.001 to 0.010 of N, balance Fe and other unavoidable impurities, and molten steel satisfying the following relational formulas 1 to 4 Casting to obtain a thin slab;
Subjecting the thin slab to rough rolling to obtain a bar;
Hot rolling the bar at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet; And
And winding the hot-rolled steel sheet at 400 to 650 ° C,
Each of the above steps is performed continuously,
A step of cold-rolling the wound hot-rolled steel sheet to produce a cold-rolled steel sheet;
Continuously annealing the cold-rolled steel sheet in a temperature range of Ac3-50 DEG C to Ac3 + 30 DEG C;
A primary cooling step of cooling the continuously annealed cold rolled steel sheet to a temperature range of Ar1 to Ar3 at an average cooling rate of 1 to 10 占 폚 / s; And
And a second cooling step of cooling the primary cooled cold rolled steel sheet to a temperature range of Mf-50 ° C to Ms + 50 ° C at an average cooling rate of 5 to 30 ° C / s,
The overlap area ratio of the cooling water injected into the bar in the finishing rolling scale breaker before and after the hot finish rolling satisfies the following relational expression 5,
Wherein the temperature deviation during the hot rolling is not more than 75 占 폚, the rolling speed deviation is not more than 60 mpm, the material variation is small, and the stretch flangeability and the water loss rate are excellent.
[Relation 1] [C]? [Cp]
[Relation 2] 1.5? [C 占 Cp] 占 1000? 3.5
[Relation 3] 165? [Ceq] x 1000? 235
[Relation 4] 50? [Ceq] / ([CxCp])? 150
[Relation 5] (X ³ -85X ² + 1800X) / 1000? 5
(Cp) in the above relational expressions 1 to 4 is 0.09-0.038C-0.013Mn-0.028Si + 0.00015Cr, [Ceq] is C + Si / 30 + Mn / 20 + 2P + 3S, , The contents of Cr, P, and S respectively represent% by weight, and in the above-mentioned relational expression 5, X means the overlap area ratio (%) of the cooling water sprayed on the steel sheet from the finish rolling scale breaker.
The method of claim 11,
Wherein the molten steel has a material deviation of less than 0.2% by weight in total of at least one selected from the group consisting of V, Mo, Cu, Ni, Zn, Se, Sb, Zr, W, Ga, A method of manufacturing a high strength cold rolled steel sheet excellent in stretch flangeability and water content.
The method of claim 11,
Wherein the continuous casting has a small material deviation at a casting speed of 4 to 8 mpm and is excellent in stretch flangeability and water content.
The method of claim 11,
Wherein the thin slabs have a small material deviation of 60 to 120 mm in thickness, and are excellent in stretch flangeability and water content.
The method of claim 11,
Further comprising the step of spraying the thin slab with cooling water at a pressure of 50 to 350 bar to remove the scale slab to a thickness of 200 mu m or less after the step of obtaining the thin slab, A method of manufacturing a high strength cold rolled steel sheet.
The method of claim 11,
A method for producing a high strength cold rolled steel sheet having a surface temperature of 900 to 1200 ° C and a surface temperature of 800 to 1100 ° C immediately after rough rolling with a small amount of material deviation and excellent stretch flangeability and water- .
The method of claim 11,
Wherein the cumulative rolling reduction during rough rolling is 70 to 90%.
The method of claim 11,
The step of spraying cooling water at a pressure of 50 to 350 bar to remove the scale to a thickness of 20 mu m or less after the step of obtaining the bar, further comprising the step of removing the scale to a thickness of 20 mu m or less, Gt;
The method of claim 11,
The method of manufacturing a high strength cold rolled steel sheet according to any one of claims 1 to 3, wherein the average passing speed in the final rolling mill during the final hot rolling is as small as 100 to 600 mpm.
The method of claim 11,
Wherein said hot-rolled steel sheet has a small material deviation of 3.0 mm or less in thickness, and is excellent in stretch flangeability and water-rejection ratio.
The method of claim 11,
The cold-rolled steel sheet according to any one of claims 1 to 3, wherein the cold-rolled steel sheet has a small material deviation with a reduction ratio of 40 to 70%.

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